This invention relates to electrochemical cells. More particularly, it is concerned with electrochemical cells employing non-aqueous electrolytic solutions and to electrolytic solutions therefor.
A particularly effective class of electrochemical cells which employs soluble or liquid cathode materials has undergone rapid development in recent years. In these cells the active cathode material is usually a fluid solvent for an electrolyte solute which provides conductivity. The active anode for these cells is usually lithium or other highly electropositive metal. During discharge the solvent is electrochemically reduced on a cathode current collector to yield ions, e.g., halide ions, which react with positive metals ions from the anode to form insoluble metal salts, e.g., metal halides.
One particular type of electrochemical cell of the foregoing class which contains a lithium anode employs a reducible liquid cathode of thionyl chloride. Typically the electrolyte solute dissolved in the thionyl chloride solvent is lithium tetrachloroaluminate. This salt is prepared from the Lewis acid aluminum chloride and the Lewis base lithium chloride. A Lewis acid is any compound capable of entering into a chemical reaction by accepting an electron pair to form a covalent bond, and a Lewis base is any compound capable of entering into a chemical reaction by donating an electron pair to form a covalent bond. Lithium/thionyl chloride electrochemical coils have proven to have outstanding weight and volume energy density, long shelf life, and unusually high power density when compared with other cells presently available.
Under unfavorable circumstances during prolonged storage or storage under extreme conditions, corrosion of the lithium anode in a cell causes a film of lithium chloride and grow on the anode sufficient to cause significant polarization at the onset of discharge of the cell. After operating at a reduced potential for a period of time, a cell may recover depending upon the severity of the condition. This initial polarization is referred to as voltage delay and has been found particularly troublesome in cells which are required to operate at current densities higher than 1 mA/cm.sup.2.
Various attempts have been made to overcome the problem of voltage delay upon startup. In one technique described in U.S. Pat. No. 4,020,240 to Carl R. Schlaikjer dated Apr. 26, 1977, a lithium clovoborate salt such as Li.sub.2 B.sub.10 Cl.sub.10 is employed as the electrolyte solute. In another technique described by J. P. Gabano in paper #27, presented at the Electrochemical Society Fall Meeting, Pittsburgh, Oct. 15-20, 1978, the electrolyte solute is a salt prepared by dissolving lithium oxide in thionyl chloride containing aluminum chloride. It is postulated that the salt produced by the reaction is Li.sub.2 (AlCl.sub.3 O AlCl.sub.3). Although the solutes of Schlaikjer and Gabano provide cells having improved startup characteristics, either they are expensive, difficult to purify or less conductive than presently used materials.